Love your blocks and your layout! For setting triangles I like that darkest magenta pink color. If you could find a blender or a batik that coordinates with that, I think it would be a good next step. Or you could do multi colors of setting triangles as already suggested. I don’t think black or white work with this quilt. IMO it would tone the quilt down too much. This quilt is like a wildflower garden and needs to make its statement all the way to the edge. If you have some bright blenders or batiks in your stash, why not lay them out with your quilt top and see if anything speaks to you? You might also get away with using a print if it’s sufficiently bright and the pattern works with what you already have.